Capacitive fingerprint sensing device

ABSTRACT

A capacitive fingerprint sensing device is provided. A sensing pixel unit includes a capacitor, a reset circuit, and a sensing capacitor. The reset circuit provides a reset voltage during a reset period to reset a voltage on the capacitor and provides an adjustment current during a sensing period to adjust a sense voltage generated on a common junction of the capacitor and the reset circuit.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefits of U.S. provisionalapplication Ser. No. 63/085,129, filed on Sep. 29, 2020. The entirety ofeach of the above-mentioned patent applications is hereby incorporatedby reference herein and made a part of this specification.

BACKGROUND Technical Field

The disclosure relates to a sensing device, and more particularly to acapacitive fingerprint sensing device.

Description of Related Art

Nowadays, fingerprint recognition is widely applied in variouselectronic products, and portable mobile devices such as mobile phonesand tablet computers are the most common. In the application offingerprint recognition in smart phones, common fingerprint recognitiondevices can be divided into optical, capacitive, ultrasonic devices, andthe like. Among these devices, capacitive fingerprint recognitiondevices are the mainstream.

Fingerprint sensors obtain the fingerprint image of a finger bydetecting the capacitance between the sensing electrode and the finger.For example, a fingerprint sensor can obtain the fingerprint image ofthe finger based on the capacitance between the sensing electrodesincluded in the sensing pixel where the finger and the ridge of thefingerprint are located and the capacitance between the sensingelectrodes included in the sensing pixel where the finger and the valleyof the fingerprint are located. Generally speaking, to be able torecognize fingerprints, the captured image must be of high resolution,and the resolution of the captured image is related to the size of thesensing pixel.

SUMMARY

The disclosure provides a capacitive fingerprint sensing device capableof providing fingerprint images of high resolution.

The capacitive fingerprint sensing device of the disclosure includes asensing pixel unit and a buffer amplifier circuit. A first terminal ofthe capacitor is coupled to a bias voltage. The reset circuit is coupledto a second terminal of the capacitor. The sensing capacitor is formedbetween a common junction of the capacitor and the reset circuit and thefinger in response to a fingerprint sensing operation of a finger. Thereset circuit provides a reset voltage to reset a voltage on thecapacitor during a reset period and provides an adjustment currentduring a sensing period to adjust a sensing voltage generated on thecommon junction of the capacitor and the reset circuit. The bufferamplifier circuit is coupled to the sensing pixel unit and amplifies thesensing voltage to generate an output voltage.

In summary, the sensing pixel unit in the embodiments of the disclosureincludes a capacitor, a reset circuit, and a sensing capacitor. Thereset circuit provides a reset voltage during a reset period to reset avoltage on the capacitor and provides an adjustment current during asensing period to adjust a sensing voltage generated on the commonjunction of the capacitor and the reset circuit. Accordingly, thecapacitive fingerprint sensing device can improve fingerprint sensingefficiency while providing fingerprint images of high resolution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a capacitive fingerprint sensing deviceaccording to an embodiment of the disclosure.

FIG. 2 is an operation timing diagram of a capacitive fingerprintsensing device according to an embodiment of the disclosure.

FIG. 3 is a schematic view of a control signal generating circuitaccording to an embodiment of the disclosure.

FIG. 4 is a schematic view of a reset voltage generating circuitaccording to an embodiment of the disclosure.

FIG. 5 is an operation timing diagram of a capacitive fingerprintsensing device according to another embodiment of the disclosure.

FIG. 6 is a schematic view of a capacitive fingerprint sensing deviceaccording to another embodiment of the disclosure.

FIG. 7 is an operation timing diagram of a capacitive fingerprintsensing device according to another embodiment of the disclosure.

DESCRIPTION OF THE EMBODIMENTS

In order to make the content of the disclosure more comprehensible, thefollowing embodiments are specifically cited as examples on which thedisclosure can indeed be implemented. In addition, wherever possible,elements/components with the same reference numbers in the drawings andembodiments represent the same or similar components.

FIG. 1 is a schematic view of a capacitive fingerprint sensing deviceaccording to an embodiment of the disclosure. Referring to FIG. 1 , thecapacitive fingerprint sensing device may include a sensing pixel unitP1 and a buffer amplifier circuit 102. The sensing pixel unit P1includes a capacitor CB, a reset circuit 104, and a sensing capacitorCF. The capacitor CB is coupled between the reset circuit 104 and a biasvoltage VB, the common junction point of the reset circuit 104 and thecapacitor CB is coupled to an input terminal of the buffer amplifiercircuit 102, and the sensing capacitor CF is formed between the commonjunction point of the capacitor CB and the reset circuit 104 and afinger F1 in response to the fingerprint sensing operation of the fingerF1.

The reset circuit 104 can provide a reset voltage VRST to reset thevoltage on the capacitor CB during the reset period, provide anadjustment current IA during the sensing period to charge the capacitorCB and the sensing capacitor CF, and adjust a sensing voltage VXgenerated on the common junction of the capacitor CB and the resetcircuit 104. The buffer amplifier circuit 102 can amplify the sensingvoltage VX to generate an output voltage Vout for the subsequent signalprocessing circuit to process the fingerprint image signal.

Accordingly, in the embodiment, a simple circuit structure is adopted toimplement the sensing pixel unit P1. The reset circuit 104 of thesensing pixel unit P1 provides a reset voltage VRST during the resetperiod to reset the voltage on the capacitor, and during the sensingperiod, an adjustment current IA is provided to adjust the sensingvoltage VX generated on the common junction of the capacitor CB and thereset circuit 104. This can effectively reduce the circuit area of thesensing pixel unit P1, provide fingerprint images of high resolution,and improve the efficiency of fingerprint sensing.

Furthermore, in the embodiment, the reset circuit 104 can be implementedby a transistor M1, but the disclosure is not limited thereto. Thetransistor M1 is a P-type transistor. The transistor M1 is coupledbetween the reset voltage VRST and the capacitor CB, and the conductionstate of the transistor M1 is controlled by a control signal TCON1. Thebuffer amplifier circuit 102 may include a transistor M2 and a currentsource I1. The transistor M2 is coupled between a power supply voltageVdd and the output terminal of the buffer amplifier circuit 102. Thecontrol terminal of the transistor M2 is coupled to the output terminalof the sensing pixel unit P1. The current source I1 is coupled betweenthe output terminal of the buffer amplifier circuit 102 and the ground.

As shown in FIG. 2 , during the reset period, the voltage value of thecontrol signal TCON1 is a voltage VC2, and the transistor M1 is in theconduction state. Meanwhile, the reset voltage VRST can reset thevoltage of the capacitor CB through the transistor M1, so that thevoltage value of the sensing voltage VX on the capacitor CB is equal tothe reset voltage VRST. The voltage value of the bias voltage VB duringthe reset period is a voltage VGL, which can be a ground voltage, forexample, but the disclosure is not limited thereto. During the sensingperiod, the voltage value of the control signal TCON1 is converted to avoltage VC1 (the voltage VC1 is less than the reset voltage VRST) with ahigher voltage level, and the bias voltage VB is converted to a voltageVGL′ with a lower voltage level, so that the transistor M1 can providethe adjustment current IA to charge the capacitor CB and the sensingcapacitor CF. The relationship among the reset voltage VRST, the biasvoltage VB, and the sensing voltage VX can be expressed as follows.

$\begin{matrix}{{VX} = {{VRST} - {\frac{CB}{{CB} + {CF1}} \times {VGL}^{\prime}}}} & (1)\end{matrix}$

Furthermore, if the capacitance value of the sensing capacitor CFcorresponding to a fingerprint peak is CF1, the capacitance value of thesensing capacitor CF corresponding to the fingerprint valley is CF1+ΔC,and the time for the adjustment current IA to charge the capacitor CBand the sensing capacitor CF is Δt, the sensing voltage VH correspondingto the fingerprint peak and the sensing voltage VL corresponding to thefingerprint valley respectively can be shown as follows.

$\begin{matrix}{{VH} = {{VRST} - {\frac{CB}{{CB} + {CF1}} \times {VGL}^{\prime}} + \frac{I \times \Delta t}{{CB} + {CF1}}}} & (2) \\{{VL} = {{VRST} - {\frac{CB}{{CB} + {CF1} + {\Delta C}} \times {VGL}^{\prime}} + \frac{I \times \Delta t}{{CB} + {CF1} + {\Delta C}}}} & (3)\end{matrix}$

During the signal output period of the sensing pixel unit P1, the biasvoltage VB can be pulled high to assist the conduction of the transistorM2. The sensing voltage VH and the sensing voltage VL can be convertedinto the output voltage Vout through a source follower including thetransistor M2 and the current source I1, and output to the subsequentsignal processing circuit to process the fingerprint image signal.

Specifically, the control signal TCON1 can be generated by the controlsignal generating circuit shown in FIG. 3 , for example. The controlsignal generating circuit may include a P-type transistor M4, a currentsource I2, a buffer B1, and an inverter A1. The P-type transistor M4 iscoupled between a reference voltage VR1 and the current source I2, thegate and drain of the P-type transistor M4 are coupled to each other,and the current source I2 is coupled between the drain of the P-typetransistor M4 and the ground. The input terminal and output terminal ofthe buffer B1 are respectively coupled to the gate of the P-typetransistor M4 and the first power input terminal of the inverter A1. Thesecond power input terminal of the inverter A1 is coupled to anotherreference voltage (the voltage VC2 is illustrated in the embodiment),the output terminal of the inverter A1 is coupled to the gate of thetransistor M1, and the input terminal of the inverter A1 receives aclock control signal S1.

Furthermore, at the drain of the P-type transistor M4, a voltage Vrefcan be generated in response to the current of the current source I2.The voltage Vref can be output to the first power input terminal of theinverter A1 through the gate of the P-type transistor M4 and the bufferB1. In the embodiment, for example, the voltage Vref can be designed tobe equal to the voltage VC1 in the embodiment of FIG. 1 , but thedisclosure is not limited thereto. The inverter A1 can be controlled bythe clock control signal S1 to output the control signal TCON1 toprovide a control signal TCON1 with a voltage value of the voltage VC2during the reset period and provide a control signal TCON1 with avoltage value of the voltage VC1 during the sensing period. The designof the control signal generating circuit of the embodiment allows thecurrent value of the current source I2 equal to the adjustment currentIA provided by the transistor M1 (reset circuit 104) during the sensingperiod, and the current adjustment can be performed in a more intuitiveand convenient way.

Note that in some embodiments, the reset voltage VRST coupled to thereset circuit 104 can also be changed so that the reset circuit 104provides the adjustment current IA during the sensing period. Forexample, the reset voltage VRST may be provided by the reset voltagegenerating circuit shown in FIG. 4 , and the reset voltage generatingcircuit may include an inverter A2. The first power terminal and thesecond power terminal are respectively coupled to the reference voltageVR1 and the reference voltage VR2, the output terminal of the inverterA2 is coupled to the transistor M1 to provide the reset voltage VRST,and the input terminal of the inverter A1 receives a clock controlsignal S2, the reference voltage VR1 is greater than the referencevoltage VR2. The inverter A2 can be controlled by the clock controlsignal S2 to output the control signal VRST to provide a reset voltageVRST with a voltage value equal to the reference voltage VR2 during thereset period and to provide a reset voltage VRST with a voltage valueequal to the reference voltage VR1 during the sensing period.

As shown in FIG. 5 , during the reset period, the voltage value of thecontrol signal TCON1 is the voltage VC2, and the transistor M1 is in theconduction state. Meanwhile, the reset voltage VRST (the voltage valueequal to the reference voltage VR2) provided by the reset voltagegenerating circuit can reset the capacitor CB through the transistor M1,so that the voltage value of the sensing voltage VX on the capacitor CBis equal to the reference voltage VR2. During the sensing period, thevoltage value of the control signal TCON1 is converted to a voltage VC1with a higher voltage level. With the voltage difference between thereset voltage VRST (the voltage value equal to the reference voltageVR1) provided by the reset voltage generating circuit and the biasvoltage VB, the transistor M1 can provide the adjustment current IA tocharge the capacitor CB and the sensing capacitor CF, and the sensingvoltage VX gradually rises with time. For example, the subsequent signalprocessing circuit coupled to the buffer amplifier circuit 102 candetermine the capacitance value of the sensing capacitor CF according tothe time required for the sensing voltage VX to rise to the presetvoltage, and the content of the fingerprint image can be furtheracquired. Alternatively, the capacitance value of the sensing capacitorCF can be determined from the voltage value of the sensing voltage VXafter the sensing period ends, so as to acquire the content of thefingerprint image.

FIG. 6 is a schematic view of a capacitive fingerprint sensing deviceaccording to another embodiment of the disclosure, compared with theembodiment in FIG. 1 . The reset circuit 104 of the embodiment of FIG. 6may further include a transistor M3 and a current source I3. Moreover,the sensing pixel unit P1 may further include a selected transistor M4,the current source I3 and the transistor M3 are connected in seriesbetween the common junction of the reference voltage VR1 and thetransistor M1 and the capacitor CB, and the selected transistor M4 iscoupled to the common junction of the transistor M1 and the capacitor CBand the gate of the transistor M2. The reference voltage VR1 is greaterthan the reset voltage VRST. In the embodiment, the conduction states ofthe transistors M1 and M3 are controlled by the control signals TCON1and TCON2, respectively, as shown in FIG. 7 . During the reset period,the control signal TCON1 is at a low voltage level to turn on thetransistor M1, the voltage value of the sensing voltage VX is reset tothe reset voltage VRST, the control signal TCON1 is at a high voltagelevel, and the transistor M3 is in an off state. During the sensingperiod, the control signal TCON1 is at a high voltage level, thetransistor M1 is in an off state, the control signal TCON2 is at a lowvoltage level, and the transistor M3 is turned on. Therefore, thecurrent source I3 can provide an adjustment current IA to charge thecapacitor CB and the sensing capacitor CF. In the embodiment, thevoltage value of the sensing voltage VX is charged to the voltage VX2during the sensing period. Moreover, the selected transistor M4 can becontrolled by the selection signal to enter the conduction state duringthe signal output period of the sensing pixel unit P1, so as to outputthe sensing voltage VX to the buffer amplifier circuit 102, the bufferamplifier circuit 102 generates the output voltage Vout according to thesensing voltage VX to the subsequent signal processing circuit toprocess the fingerprint image signal. During the signal output period,the voltage value of the bias voltage VB can be pulled up to assist theconduction of the transistor M2 and convert the sensing voltage VX intothe output voltage Vout.

In summary, the sensing pixel unit with a simple circuit structure isimplemented in the embodiments of the disclosure. The sensing pixel unitmay include a capacitor, a reset circuit, and a sensing capacitor. Thereset circuit can provide a reset voltage to rest the voltage on thecapacitor during the reset period and provide an adjustment currentduring the sensing period to adjust the sensing voltage generated on thecommon junction of the capacitor and the reset circuit. Accordingly, thecapacitive fingerprint sensing device can improve fingerprint sensingefficiency while providing fingerprint images of high resolution.

What is claimed is:
 1. A capacitive fingerprint sensing device,comprising: a sensing pixel unit, comprising: a capacitor, wherein afirst terminal of which is coupled to a bias voltage; a reset circuitcoupled to a second terminal of the capacitor; and a sensing capacitorformed between a common junction of the capacitor and the reset circuitand the finger in response to a fingerprint sensing operation of afinger, wherein the reset circuit provides a reset voltage to reset avoltage on the capacitor during a reset period and provides anadjustment current during a sensing period to adjust a sensing voltagegenerated on the common junction of the capacitor and the reset circuit;and a buffer amplifier circuit coupled to the sensing pixel unit andamplifying the sensing voltage to generate an output voltage, whereinthe reset circuit comprises a first transistor coupled between the resetvoltage and the second terminal of the capacitor and controlled by acontrol signal to provide the reset voltage during the reset period andprovide the adjustment current during the sensing period, wherein thebias voltage is converted from a first bias voltage to a second biasvoltage when entering a signal output period to turn on the firsttransistor.
 2. The capacitive fingerprint sensing device according toclaim 1, further comprising: a control signal generating circuit coupledto the control terminal of the first transistor to generate the controlsignal, wherein the control signal generating circuit comprises: aP-type transistor, wherein a source terminal of the P-type transistor iscoupled to a first reference voltage, and a drain and a gate of theP-type transistor are coupled to each other; a current source coupledbetween the drain of the P-type transistor and a ground; a buffercoupled to the gate of the P-type transistor and outputting a firstvoltage according to a gate voltage of the P-type transistor; and aninverter, wherein a first power input terminal of the inverter iscoupled to an output terminal of the buffer, and a second power inputterminal of the inverter is coupled to a second voltage, the inverter iscontrolled by a clock control signal to output the second voltage duringthe reset period and output the first voltage during the sensing periodto control the first transistor to provide the reset voltage during thereset period and provide the adjustment current during the sensingperiod.
 3. The capacitive fingerprint sensing device according to claim2, wherein the reset voltage is greater than the first voltage.
 4. Thecapacitive fingerprint sensing device according to claim 1, wherein thereset voltage has a first voltage value during the sensing period andhas a second voltage value during the reset period, and the firstvoltage value is greater than the second voltage value.
 5. Thecapacitive fingerprint sensing device according to claim 4, furthercomprising: a reset voltage generating circuit coupled to the firsttransistor to generate the reset voltage, and the reset voltagegenerating circuit comprises: an inverter, wherein a first power inputterminal of the inverter is coupled to a first reference voltage, asecond power input terminal of the inverter is coupled to a secondreference voltage, and the inverter is controlled by a clock controlsignal to output the first reference voltage during the sensing periodand output the second reference voltage during the reset period, whereinthe first reference voltage has the first voltage value, and the secondreference voltage has the second voltage value.
 6. A capacitivefingerprint sensing device, comprising: a sensing pixel unit,comprising: a capacitor, wherein a first terminal of which is coupled toa bias voltage; a reset circuit coupled to a second terminal of thecapacitor; and a sensing capacitor formed between a common junction ofthe capacitor and the reset circuit and the finger in response to afingerprint sensing operation of a finger, wherein the reset circuitprovides a reset voltage to reset a voltage on the capacitor during areset period and provides an adjustment current during a sensing periodto adjust a sensing voltage generated on the common junction of thecapacitor and the reset circuit; and a buffer amplifier circuit coupledto the sensing pixel unit and amplifying the sensing voltage to generatean output voltage, wherein the reset circuit comprises: a firsttransistor coupled between the reset voltage and the second terminal ofthe capacitor, controlled by a first control signal to provide the resetvoltage during the reset period, and turned off during the sensingperiod a current source; and a second transistor coupled to the currentsource between a reference voltage and a common junction of thecapacitor and the reset circuit and controlled by a second controlsignal to provide the adjustment current during the sensing period,wherein the reference voltage is greater than the reset voltage.
 7. Thecapacitive fingerprint sensing device according to claim 1, wherein thebuffer amplifier circuit comprises: a second transistor, wherein a firstterminal of the second transistor is coupled to a power supply voltage,a second terminal of the second transistor is coupled to an outputterminal of the buffer amplifier circuit, and a control terminal of thesecond transistor is coupled to an output terminal of the sensing pixelunit; and a current source is coupled between the second terminal of thesecond transistor and the ground.
 8. The capacitive fingerprint sensingdevice according to claim 1, further comprising: a selection transistorcoupled between the common junction of the capacitor and the resetcircuit and the buffer amplifier circuit and controlled by a selectionsignal to be turned on during a signal output period.